Color management system for converting computer graphic images to film images

ABSTRACT

A system for efficiently converting computer graphics images to film images with accurate color management is described. The system involves the creation of a direct mapping of chromaticity and intensity data from the values used to generate images on a computer monitor to the values used to display the images on projected motion picture film.

The present invention relates to digital film production and, moreparticularly, to techniques for efficiently and accurately convertingcolor computer graphics images to film images.

BACKGROUND OF THE INVENTION

When creating a computer-animated motion picture, the animation teamuses computers to create, view, and manipulate images used in the motionpicture. The animation team makes judgments concerning the desiredappearance of the images, and manipulates the images, based on how theimages appear when displayed on the computer monitors.

The motion picture ultimately will be displayed to a theater audienceusing conventional motion picture film and projectors. Accordingly, theimage data stored on computer must be converted into film images forprojection in a theater.

Digital film recorders are used to convert the original,computer-generated images created by the animation team into images onphotosensitive motion picture film. Conventional digital film recordersuse a light source (such as a laser) to expose each frame of the film asnecessary to produce the desired image in the frame. The film is thenadvanced to the next frame and the process is repeated. When a strip offilm has been recorded, it is sent to a laboratory for development intoa color negative and, ultimately, a positive print.

To create high quality computer-generated films, film producers mustmake certain that the ultimate appearance of the motion picture that isprojected and viewed by an audience matches the appearance desired bythe creative team. The color of each location on the exposed film shouldmatch, as closely as possible, the color of the corresponding pictureelement (“pixel”) on the display device used by the artist who createdthe image. To produce a visually accurate color image using a digitalfilm recorder, the locations on a frame of film that correspond to eachpixel of an image must be exposed precisely. The calculation of thisexposure (for each primary component, red, green and blue) depends onthe nature of the source and the characteristics of the film used.

Producing a film image that corresponds visually to the original imagecreated and/or stored on a computer is not as straightforward as itmight initially appear. Difficulties arise because the color densitiesproduced on a film generally do not correspond linearly to the RGB colorvalues that displayed on the computer monitor. The color densitiesactually produced on the film are affected by a variety of factors,including chemical characteristics of the film itself. For example, dueto chemical characteristics of the film, the density of a particularcolor produced on the film by a beam of light generally will not varylinearly with the intensity of light used to expose the film for a giventime duration. Temperature, film type, characteristics of a light sourceused in the recorder and noise generated by the system can affect thecolor density values actually produced.

Similarly, the stored value used to produce a particular measureddensity for one color component in a neutral tone generally will notproduce the same measured density in color. For example, if thecombination of red, green and blue code values X1, Y1 and Z1 producemeasured red, green and blue densities R1, G1, and B1, then the red codevalue X1 used in combination with different green and blue code values(i.e., Y2 and Z2) generally will not produce the density R1.

The monitor itself has nonlinear, characteristics. The well-known gammacorrection process is used to correct for nonlinearities in specificmonitors. Nevertheless, even a monitor's characteristics tend to degradeover time. Monitors operate in RGB color spaces in which colors arecreated by mixing proportions of Red, Green and Blue light. Monitorsfrom different suppliers may use different phosphors and an individualmonitor itself will age. This is equivalent to different or graduallychanging color spaces. All of these problems and complexities suggest tothose skilled in the field the complex color management schemesnecessary.

Another problem is that the film cannot reproduce all the colorsreproducible on the monitor (the converse is also true, but lessimportant since we're only trying to mimic the monitor). This phenomenonis referred to as “gamut mismatch,” and treating such cases in aconsistent manner is difficult. One approach would be to take all thecolors that are outside the film's gamut and map them to the closest (insome sense) point on the surface of the film's gamut. Colors which areinside the gamut to start with would be left alone. This approach,however, results in abrupt color changes which result in bandingartifacts in the final image.

Industry literature generally teaches that solutions to theabove-described problems must be complex. “Historically, managing colorhas been a very time consuming and costly process in the printing,prepress, and film industries.” Has & Newman, Color Management: CurrentPractice and The Adoption of a New Standard. “Color is an immenselycomplex subject, one that draws on concepts and results from physics,physiology, psychology, art, and graphic design. The color of an objectdepends not only on the object itself, but also on the light sourceilluminating it, on the color of the surrounding area, and on the humanvisual system.” See Foley and Van Dam, Computer Graphics Principles andPractice (Second Edition 1996).

In accordance with such theory, conventional approaches to producingcolor images on film that match the originally created computer graphicsimages have been complex. For example, some conventional approachesinvolve the use of complex models of the film development process tochange the primary color component values derived from the computergraphics images into values that will produce a similar visual result onfilm. Other conventional approaches arrive (by trial and error and muchmanual tweaking) at a transfer curve for each channel such that thecolors on the screen look acceptable. Such methods do not allow forfine-grained, color-by-color matching. In any event, it is generallyunderstood that brightness, or intensity, values that are used inconnection with the display of images on a computer screen cannot beconverted to the film media, as they are irrelevant to the visualperception of the film in the theater environment. In the theaterenvironment, so the reasoning goes, the psychophysical perception ofvarious colors by the audience is affected by the darkness of thetheater and numerous other subtle factors that are difficult orimpossible to account for in advance. It is generally understood thatthe intensity value of each color component must be determined by visualinspection in the new medium (i.e., the film projected in a theaterenvironment).

Conventional approaches to dealing with the above matter typicallyaddress the reproduction of color on print media, which cannot reproducecolors as well as monitors can. Most importantly, the dynamic range,(ratio of maximum to minimum brightness values) is substantially lowerthan that of monitors. Much attention is therefore focused on theproblem of compressing the dynamic range. Film, on the other hand,offers a dynamic range that is comparable (or even larger) than that ofmonitors, an issue which has not been addressed adequately in theliterature.

Efficient and accurate techniques for converting color computer graphicsimages to film images would be highly desirable.

SUMMARY OF THE INVENTION

The present invention involves a color management system for efficientlyand accurately converting computer graphics images to film images. Incertain embodiments, the system includes at least three steps:determining an RGB-to-XYZ mapping for the monitor; measuring theRGB-to-XYZ mapping applicable to the film recording; and creating anRGB-to-RGB mapping for operating on the image colors displayed on themonitor to create accurate color densities on the film. In manyembodiments, each mapping uses color spaces that incorporate bothchromaticity and intensity data for each color component.

As will be understood by those skilled in the art based on the presentdisclosure, a wide variety of embodiments of the present invention existand fall within the scope of the claims set forth herein. The scope ofthe present invention is defined by the claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows one embodiment of a digital film production system inconnection with which color management according to the presentinvention is useful.

FIG. 2 shows a series of steps for performing color management accordingto the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

A diagram of one example of a digital film production system forrecording computer-generated images onto motion picture film isillustrated at 10 in FIG. 1. Embodiments of the present invention areuseful when performing color management in connection with anyconventional film production system, including film production system10.

In FIG. 1, computer system 102 preferably is a workstation, although anyconventional computer system may be used. In addition to otherconventional components, computer system 102 comprises a conventionalmonitor 104 used to view images during the film production process.Creative personnel 101 create, manipulate and modify images on computersystem 102, viewing the images on monitor 104 and basing their creativejudgments regarding color in part on the appearance of the images whendisplayed on monitor 104.

After images have been created and modified using computer system 102,the images are printed onto motion picture film using a film recorder106. Film recorder 106 is a conventional digital film recorder,preferably a laser recorder that exposes film according to specified RGBvalues. It will be appreciated by those skilled in the art that thepresent invention is also useful in connection with CRT and EBR(electron beam) recorders.

The recorded filmstrips are then sent to a photographic laboratory(“lab”) 108 for processing. In a conventional manner, the filmstrips aredeveloped to create a negative, which is used to create a positivefilmstrip 110.

Filmstrip 110 is then viewed using a conventional motion picture filmprojector 112.

In the past, in order to address the problem of color management withrespect to chromaticity alone, film producers have created a look-uptable 113 to convert the color data stored on the computer into colordata that can be used by film recorder 106 to expose a strip of film andused complicated models 115 to account for nonlinearities duringlaboratory processing. The film has then been viewed using projector 112to determine appropriate intensity levels for each color component for agiven motion picture. Certain embodiments of the present inventioneliminate the need for such complexity.

In accordance with the present invention, color management is performedin connection with converting the computer graphics images created oncomputer system 102 to the film images printed on filmstrip 110 that areprojected for a viewing audience using projector 112. Certainembodiments of the present invention looks at the system “end-to-end,”from the computer graphics image input to the projected image output,and involve the creation of a monitor-to-film (“MFM”) mapping 120 forthe system using color spaces that incorporate both chromaticity andintensity data for each color component.

Color management in accordance with certain embodiments of the presentinvention involves three main steps, which need not be performed in theorder listed below. The first step (step 202) is to determine a firstRGB-to-XYZ mapping applicable to the monitor. This first RGB-to-XYZmapping preferably uses color spaces incorporating both chromaticity andintensity data for each color component.

The XYZ color space is a way of representing color information in deviceindependent terms in a manner that incorporates both intensity andchromaticity data for each color component. The basis of deviceindependent color representation is usually the CommissionInternationale De L'Eclairage (“CIE”) XYZ space. It will be apparent tothose skilled in the art, based on the present disclosure, that colorspaces other than the CIE XYZ space may be used as the XYZ color spacefor purposes of the present invention.

Certain embodiments' of the present invention use an adaptation of theATD space proposed by Lee Guth. Other embodiments use other perceptuallyuniform spaces, like CIELab or CIELuv. Those skilled in the art willrecognize that the color space in which measurements are made (XYZ) andthe computation space (ATD, preferably) are distinct. As will beapparent to those skilled in the art, the choice of computation space isimportant for the treatment of out-of-gamut colors.

The second step (step 204) is to make appropriate measurements to createa second RGB-to-XYZ mapping applicable to the film recording. The secondRGB-to-XYZ mapping preferably uses color spaces incorporating bothchromaticity and intensity data for each color component. To accomplishthis objective, one records a strip of 1000 frames of flat color patchescalculated to span the entire RGB space (10 steps along each axis).Next, one projects these frames on to a screen, and points aspectrophotometer (preferably, a Spectrascan PR-650) at the center ofthe patch. One reads the spectrum (101 samples in 4 nm steps from 380 to780 nm), corrects it for the projector bulb spectrum (i.e. divide bytest projector spectrum and multiply by theater projector spectrum), andconverts to an XYZ triple by integration. (This integration process iswell-defined by CIE). One thus obtains asset of 1000 readings, whereeach reading consists of a pair of triples: the RGB triple used torecord the patch, and the resulting XYZ triple. The XYZ values areconverted finally to the internal computation space (ATD).

The third step (step 206) is to create an RGB-to-RGB mapping, preferablyincorporating both chromaticity and intensity data for each colorcomponent, for operating on the image colors displayed on the monitor tocreate accurate color densities on the projected film.

The present invention also addresses the issue of gamut mismatch. Thebasic idea is to “pull in” colors along a line connecting the gray (orachromatic) axis and the color to be reproduced. This line intersectsboth film and monitor gamut surfaces. If the monitor surfaceintersection is nearer than the film surface intersection, one need donothing, because all colors along this line are reproducible on film. Ifnot, one smoothly (using a quadratic function) remaps colors along theline such that the monitor surface intersection color is mapped to thefilm surface intersection color.

Although the present invention has been described in connection withcertain specific embodiments, it will be clear to those skilled in theart that the inventive features of the present invention are applicableto other embodiments as well. All of such embodiments are intended tofall within the scope of the present invention.

Based on the present disclosure, those skilled in the art willunderstand that the present invention has broad applicability inconnection with the use of computers during the production or processingof film. For example, embodiments of the present invention are useful inconnection with computer generation of entirely synthetic images for 2Dor 3D animation. Embodiments of the present invention are also useful inconnection with the use of computers to modify live action film, eitherfor touch up or to add special effects.

What is claimed is:
 1. A color management system for converting computergraphics images to film images, comprising; determining a first RGB toXYZ mapping applicable to the monitor, said first RGB to XYZ mappingusing color spaces incorporating chromaticity and intensity data foreach color component; recording a plurality of film images onto filmusing a film recorder driven with specified RGB values; outputting theplurality of film images with a projection system as a plurality ofprojected images; recording a plurality of light samples from theplurality of projected images; determining a second RGB to XYZ mappingapplicable to the film recorder in response to the plurality of lightsamples and the specified RGB values, the second RGB to XYZ mappingusing color spaces incorporating chromaticity and intensity data foreach color component; and determining an RGB to RGB mapping,incorporating chromaticity and intensity data for each color component,for operating on the image colors displayed on the monitor to createaccurate color densities on the film in response to the first RGB to XYZmapping applicable to the monitor and in response to the second RGB toXYZ mapping applicable to the film recorder.
 2. A method for a computersystem comprises: determining a first RGB to XYZ color space mapping,wherein the first RGB to XYZ color space mapping is associated with amonitor and wherein the XYZ color space is device independent;determining a second RGB to XYZ color space mapping, wherein the secondRGB to XYZ color space mapping is associated with a film recorder anddetermined in response to a plurality of images that are recorded withthe film recorder and sampled; determining a smooth remapping from amonitor gamut in the XYZ color space to a film recorder gamut in the XYZcolor space; and determining an RGB to RGB mapping between monitordriving RGB values and film recorder driving RGB values, in response tothe first RGB to XYZ color space mapping, to the second RGB to XYZ colorspace mapping, and to the smooth remapping.
 3. The method of claim 2,wherein the XYZ color space comprises chromaticity and intensity data;and wherein the smooth remapping from the monitor gamut to the filmrecorder gamut includes intensity data.
 4. The method of claim 2,wherein determining a smooth remapping from the monitor gamut and thefilm recorder gamut comprises: identifying a first color in the XYZcolor space; when the first color is within the monitor gamut but notwithin the film recorder gamut, the method further comprises:determining a first distance between a gray location and a color on asurface of the monitor gamut in a direction along a line from the graylocation through the first color; determining a second distance betweena gray location and a color on a surface of the film recorder gamut inthe direction along the line; and remapping colors ranging from the graylocation to the color on the surface of the monitor gamut to colorsranging from the gray location to the color on the surface of the filmrecorder gamut.
 5. The method of claim 2, further comprising storing theRGB to RGB mapping in a look up table.
 6. The method of claim 5 furthercomprising: receiving RGB values associated with the monitor; mappingthe RGB values associated with the monitor to RGB values associated withthe film recorder via the look up table; and writing an image to filmwith the film recorder using the RGB values associated with the filmrecorder.
 7. The method of claim 2 wherein the smooth remapping from themonitor gamut and the film recorder gamut occurs at different intensityvalues.
 8. The method of claim 2 wherein a first color and a secondcolor in the monitor gamut but not within the film recorder gamut arenot mapped to the same color within the film recorder gamut.
 9. Themethod of claim 2 wherein determining the second RGB to XYZ color spacemapping associated with a film recorder comprises: writing recording aplurality of images to film with a film recorder using RGB values;projecting the plurality of images written to the film with a projector;determining colors for the projected images; and converting colors ofthe projected images into XYZ color space.
 10. A color management systemcomprises: a computer system including a monitor configured to display aseries of images, wherein the computer system is configured to drive themonitor with RGB signals; a film recorder configured to record theseries of images onto film, wherein the film recorder is configured tobe driven with RGB signals; a processor coupled to the computer systemand to the film recorder, the processor configured to map from RGB drivevalues for the monitor to RGB drive values for the film recorder;wherein a mapping from RGB drive values for the monitor to RGB drivevalues for the film recorder are determined in response to a mapping,between the RGB drive values for the monitor and chromaticity andintensity values for the monitor, in response to a mapping, between theRGB drive values for the film recorder and the chromaticity andintensity values associated with images on the film that are written bythe film recorder, and in response to a mapping; from a monitor gamut inchromaticity and intensity color space to image gamut of the film in thechromaticity and intensity color space.
 11. The color management systemof claim 10, wherein the computer system comprises a work station. 12.The system of claim 10, wherein the film recorder comprises a digitalfilm recorder configured to expose the film with light at predeterminedintensity levels corresponding to specified RGB values.
 13. The computersystem of claim 10, wherein a color is within the monitor gamut but notwithin the image gamut.
 14. The computer system of claim 10, wherein acolor is within the film recorder gamut but not within the monitorgamut.
 15. The computer system of claim 10, wherein the mapping from theRGB drive values for the monitor to RGB drive values for the filmrecorder compensates for different intensities for each color component.16. The color management system of claim 10 wherein a first color thatis within the monitor gamut but not within the film recorder gamut isremapped into a first color that is within the film recorder gamut;wherein a second color that is within the monitor gamut but not withinthe film recorder gamut and that is co-linear to a line drawn from agray color to the first color, is remapped into a second color that iswithin the film recorder gamut; and wherein the first color is differentfrom the second color.
 17. The method of claim 10 wherein the mappingfrom the RGB drive values for the monitor to RGB drive values for thefilm recorder associates an RGB drive value for the monitor with an RGBdrive value for the film recorder, and wherein at least one colorcomponent of the RGB drive value for the monitor is larger than acorresponding color component of the RGB drive value for the filmrecorder.
 18. The method of claim 10 wherein the processor configured tomap from RGB drive values for the monitor to RGB drive values for thefilm recorder comprises a look-up-table.
 19. The method of claim 10wherein the values associated with images on the film written by thefilm recorder are determined by illuminating the images on the film andsampling outputs of the images while the film is illuminated.